The present invention generally relates to a silicon crystal evaluation method, and more particularly to a silicon crystal evaluation method directed to predicting how much oxygen precipitates are formed during a specific heat treatment for a device using a silicon crystal which has an unknown thermal history and to discriminating silicon crystals having an identical oxygen impurity concentration and having different thermal histories from each other. Further, the present invention is directed to providing a silicon crystal evaluation method capable of accurately obtaining the concentration of isolated interstitial oxygen impurities in a silicon crystal and evaluating precipitated oxygen impurities in a silicon crystal on the basis of a standard with respect to the state (morphology) and content of precipitated impurities (precipitate defects). Moreover, the present invention is concerned with a method of fabricating a semiconductor integrated circuit device using the silicon crystal evaluation method.
There is known a phenomenon in which oxygen impurities contained in a silicon crystal in a thermally unstable morphology (a supersatulated state, for example) are morphologically changed during a heat treatment in semiconductor device processing. Conventionally, it is required to precisely predict the above-mentioned phenomenon by a crystal evaluation which is carried out before the semiconductor device fabrication process. Oxygen precipitates contained in a silicon crystal have the function of absorbing impurity atoms, such as Fe, Ni and Co, which are not desirable to be contained in a device active region of the silicon crystal. This function is known as the gettering effect.
Conventionally, there is no evaluation method directed to evaluating morphological differences of oxygen impurities contained in as-grown crystals before the semiconductor device fabrication process and knowing how many oxygen precipitation nuclei are contained before the fabrication process. Slight morphological differences of oxygen impurities contained in (as grown) crystals cause great differences in the amount of precipitated oxygen after the heat treatment during the fabrication process and thus cause remarkable problems frequently.
Oxygen impurities can exist in various states due to various thermal histories of crystals even when the concentration of oxygen impurities in silicon crystals obtained immediately after the crystal production is the same. It should be noted that the above has been known only experimentially. In other words, there is no special measurement means for quantitatively measuring various states of oxygen impurities contained in crystals.
Although it is possible to obtain the concentration of all oxygen atoms contained in an as-grown silicon crystal by using a conventional room temperature infrared absorption spectra analysis, it is impossible to identify morphological differences of oxygen precipitation nuclei in the as-grown silicon crystal, particularly due to the thermal history during silicon crystal production processing. That is, it is impossible to specify the starting point of the oxygen precipitation phenomenon arising from a heat treatment (annealing, for example) during the device fabrication process, or initial morphological differences of oxygen impurities contained in crystals. For the above-mentioned reason, even when crystals having an identical oxygen concentration is subjected to the same heat treatment during device processing, the content of oxygen precipitation nuclei is different for different crystals due to the respective thermal histories thereof during the crystal production process.
As is well known, the aforementioned gettering effect depends on the total amount (number) of oxygen precipitate defects. The gettering effect is zero when there is no precipitate defect. On the other hand, a large degree of the gettering effect is expected when there is a large number of oxygen precipitate defects. Further, there is a variety of morphology of oxygen precipitates. Thus, the gettering effect depends on morphology of precipitates (related to the structure and size) as well as the content (concentration) of precipitates related to individual states thereof.
The inventors have been proposed a silicon crystal evaluation method by Japanese Laid-Open Patent Application No. 63-167907. Except for this silicon crystal evaluation method, the inventors do not know a method for detecting a morphological change of oxygen impurities (formation of oxygen precipitation nuclei and growth thereof) contained in an as-grown crystal. This change depends on the thermal history of an as-grown crystal or a light heat treatment equivalent to the thermal history of the as-grown crystal.
Conventionally, it is possible to evaluate a great morphological change of oxygen impurities contained in a crystal resulting from a special thermal history or a heavy heat treatment by a photoluminescence method (see M. Tajima et al., Appl. Phys. Lett., 43, 1983, pp.274). However, the photoluminescence method is not capable of detecting a morphological change of oxygen impurities (formation of oxygen precipitation nuclei) due to a relatively light heat treatment. By using an infrared absorption spectra analysis, it is also possible to detect a morphological change of oxygen impurities at a time when oxygen precipitation is in an advanced stage (see F. Shimura et al., Appl. Phys. Lett., 46, 1985, pp. 941). Except for this case, it is presently impossible to detect oxygen precipitation nuclei (see F. shimura et al., Appl. Phys. Lett., 46, 1985, pp. 941, or M. Tajima et al., Appl. Phys. Lett., 51, 1983, pp. 2247).
In addition, it is said that the room-temperature infrared absorption spectra method does not provide a large amount of evaluation information and many species of evaluation information. The aforementioned Japanese application No. 63-167907 is not capable of measuring, at a temperature equal to or lower than 10K, absorption spectral peaks of L1, L2, L3 and M which are some peaks to be measured. In addition, the peak intensity depends on temperature greatly at temperatures higher than 10K, and thus the measurement temperature must be set within a narrow allowable temperature range equal to 0.05K. However, it is very difficult to regulate the measurement temperature.
As has been mentioned hereinbefore, there is no simple silicon crystal evaluation method directed to predicting oxygen precipitation. Conventionally, oxygen precipitation is predicted by measuring the content of all oxygen impurities by means of the aforementioned room-temperature infrared spectra analysis. According to the room-temperature infrared spectra analysis, the concentration of all oxygen impurities is quantitatively measured by utilizing a room-temperature 1106.+-.1 cm.sup.-1 peak which is an absorption peak resulting from oxygen atoms in a silicon crystal. As will be described later, this method does not accurately provide the concentration of oxygen precipitates.
It will be noted that the aforementioned differences of the oxygen precipitation content arise from the fact that there is a variety of morphology of oxygen impurities contained in a silicon crystal even if the oxygen concentration is constant and that conventional methods, particularly the room-temperature infrared spectra analysis, are not capable of measuring morphological differences of oxygen impurities in silicon crystals.
As described above, there is experientially known the phenomenon in which oxygen precipitation is based on conditions for producing crystals, especially the thermal histories during the crystal production process and there is a variety of morphology (states) of oxygen impurities in crystals (which are respectively starting points of the device processing) due to the differences in thermal histories during the crystal production process. However, there is nothing directed to easily obtaining physical values which reflect the differences of morphology of oxygen impurities contained in silicon crystals.
The aforementioned infrared absorption spectra analysis utilizing the room-temperature 1106.+-.1 cm.sup.-1 peak cannot provide accurate physical values which reflect the morphological differences of oxygen impurities in silicon crystals due to the following reasons.
A description will now be given of a variety of morphology of oxygen impurities contained in silicon crystals in order to facilitate understanding the problems of the room-temperature infrared absorption spectra analysis. A typical morphology (state) of oxygen impurities contained in silicon crystals is isolated interstitial oxygen atoms which are located in a mutually isolated state. Hereinafter, this is represented by Oi. Oi generates an impurity infrared absorption peak at a wavenumber of 1106.+-.1 cm.sup.-1 at room temperature. This peak is called an Oi peak. Normally, in a (as-grown) state before precipitates are formed, almost all oxygen impurities exist in the form of isolated interstitial oxygen impurities (Oi). Thus, it is possible to quantitatively obtain the oxygen concentration [Oi]of as-grown crystal by measuring the intensity of an Oi peak as shown in FIG. 1. On the other hand, precipitated oxygen impurities (oxygen precipitate defects) generate the Oi peak no longer. Thus, the intensity of the Oi peak decreases by a level corresponding to the content of oxygen precipitate defects formed from some of all the oxygen impurities [Oi]. By using a decreased peak intensity level, it is possible to predict how much oxygen impurities are precipitated or how much precipitate defects exist.
However, this method has the following problems. As shown in FIG. 2, a state of precipitate defects (called P1 state) generate an impurity infrared absorption peak (called P1 peak) at a wavenumber (approximately 1106.+-.1 cm.sup.-1) which is almost the same as that of the Oi peak. The P1 peak overlaps the Oi peak so that they are not separated from each other. As a result, a decreased intensity of the Oi peak due to the formation of oxygen precipitates is not evaluated correctly. If the entire overlap of the P1 peak and the Oi peak is considered as the Oi peak without noticing the presence of the P1 peak, the oxygen precipitate defects in the P1 state are neglected so that the content of all precipitate defects (the amount of precipitated Oi) is underestimated. Currently, it is very difficult to separate the P1 peak from the Oi peak by the room-temperature infrared absorption spectra analysis. From this viewpoint, at present, errors introduced by this analysis cannot help being accepted. That is, the current precipitate defect evaluation method cannot indicate the accurate content of precipitate defects.
Further, the conventional precipitate defect evaluation method has a serious problem described below. There are many morphological differences of precipitate defects in view of the structure and the size thereof. It is generally considered that the gettering effect depends, to some extent, on the total content of precipitate defects (the precipitated Oi content). In addition, the gettering effect greatly depends on what state of precipitates is contained and how much the precipitates are contained. Conventionally, it is possible to observe the shape and/or size of precipitate defects by chemically etching the precipitate defects and measuring the etched precipitate defects by a microscope. However, the size, shape or density of precipitate defects change by the etching condition. For example, by changing the concentration or composition of an etchant or etching time, precipitates which have not been observed (counted) appear, or precipitates which have been observed disappear. Moreover, the number of precipitates is counted by the naked eye. Thus, there is a problem that the counted results greatly depend on who counts the precipitates. In addition, there are fine differences related to the structure of precipitate defects, which cannot be observed by the naked eye. In some cases, the fine differences considerably affect the gettering effect.